Process for fabricating polypropylene sheet

ABSTRACT

A process for production of a monolithic article from a web of fibres of oriented polypropylene polymer, comprising the steps of subjecting the web to elevated temperature and pressure sufficient to melt a proportion of the polymer and compact it, and then cooling the compacted web, wherein an accelerated rate of cooling is employed down to 100° C. The process is of particular benefit when the weight average molecular weight (M w ) of the fibres is 250,000 or below. The resultant articles have good stiffness and strength, yet with reasonable ductility. Similar articles cooled slowly are brittle.

[0001] The present invention relates to polymer sheet materials madefrom oriented olefin polymer fibres or tapes, and in particular animproved process for making such materials.

[0002] In recent years, developments have been made in processes forcompacting polyolefin fibres in order to make sheets of high stiffnessand strength. Two-step compaction processes for melt-spun fibresemploying high compaction pressures are well known. An example isdisclosed in GB 2253420A, in which an assembly of fibres of an orientedpolymer is hot compacted in a two-step process to form a sheet havinggood mechanical properties. The process involves an initial step inwhich the fibres are brought to and held at the compaction temperaturewhilst subject to a pressure sufficient to maintain the fibres incontact, and thereafter compacted at a high pressure (40-50 MPa) for afew seconds (the compaction pressure). In this process a proportion ofthe fibre surfaces, generally from 5 to 10 percent by weight, melt andsubsequently recrystallise on cooling. This recrystallise phase bindsthe fibres together, resulting in good mechanical properties of thefinal sheet.

[0003] It is mentioned in GB 2253420A that the process can be applied tomany types of oriented polymer including polyester and PEEK (polyetherether ketone) but that preferred polymers are oriented polyolefins.Polyethylene is the only polyolefin mentioned, and is used in all of theexamples.

[0004] In WO 98/15397, an improvement to the above process is disclosedin which an assembly of melt-formed polyolefin fibres is maintained inintimate contact at elevated temperature sufficient to melt a proportionof the fibres, whilst being subjected to a compaction pressure of nogreater than 10 MPa. This single-step, low-pressure process alsoproduces products having excellent mechanical properties. If wished thefibres may have been subjected to a prior crosslinking process,preferably an irradiation crosslinking process comprising irradiatingthe fibres with an ionising radiation in an inert environment containingalkylene or diene compounds, and then an annealing step comprisingannealing the irradiated polymer at an elevated temperature, again in aninert environment containing alkyne or diene compounds.

[0005] In GB 2253420A it is stated that “the hot compacted materials arepreferably cooled to ambient temperature under controlled conditions.Rapid cooling is less preferred. The most convenient technique is toallow the compacts to stand in the air until they have cooled to ambienttemperature.” The examples of GB 2253420A do not mention cooling rate.

[0006] In the examples of WO 98/15397 the compaction temperature andpressure were applied and the assembly was cooled under the compactionpressure to 100° C. by passing a mixture of air and water through theheating platens. At this point the assembly was removed from the pressand cooled to room temperature in air with no pressure applied. Coolingrate is not mentioned.

[0007] In Plastics, Rubber and Composites Processing and Applications,1998, Vol 27, No. 4, pgs 167-171, specifically in relation topolyethylene it was stated that “the final cooling rate does notsignificantly affect the structure or properties of the final compactedsheet: quenched samples have been measured to have almost identicalproperties to slow cooled samples.”

[0008] Despite the above, we now have evidence to suggest that thecooling rate can have a significant effect on the final properties ofcompacted articles formed from fibres of oriented homopolymer orcopolymer material.

[0009] In particular, we have discovered that unlike polyethylene, inthe case of polypropylene the cooling rate has a significant effect onthe final properties of the compacted sheet. Specifically, fastercooling rates result in the improvement of some properties, inparticular ductility, and related properties such as peel strength.

[0010] Accordingly, the present invention provides a process forproduction of a monolithic article from a web of fibres of orientedpolypropylene homopolymer or copolymer, comprising the steps ofsubjecting the web to elevated temperature and pressure sufficient tomelt a proportion of the polymer and compact it, and then cooling thecompacted web, wherein an accelerated rate of cooling is employed downto a lower temperature and in which said lower temperature is apredetermined amount below the recrystallisation temperature of thefibres.

[0011] In a particular aspect, the present invention provides a processfor production of a monolithic article from a web of fibres of orientedpolypropylene homopolymer or copolymer, comprising the steps ofsubjecting the web to elevated temperature and pressure sufficient tomelt a proportion of the polymer and compact it, and then cooling thecompacted web, wherein an accelerated rate of cooling is employed downto 100° C.

[0012] The fibres can be made by any suitable process, for example bysolution or gel or melt forming, but most preferably by melt forming.

[0013] The term “fibres of oriented polypropylene homopolymer orcopolymer” is used herein to denote all elongate elements which comprisepolypropylene. They may be in the form of strands or filaments. They maybe in the form of bands, ribbons or tapes, formed for example byinitially slitting melt formed films. Whatever their form the fibres maybe laid in a non-woven web for the process of the invention.Alternatively they may be formed into yarns comprising multiple fibres,or used in the form of a monofilament yarn. The fibres are usuallyformed into a fabric by weaving or knitting. Optionally the fibres mayhave been subjected to a crosslinking process, as described in WO98/15397. Woven fabrics may comprise only fibres in the form of strandsor filaments, or they may comprise a mixture of fibres in the form ofstrands or filaments and fibres in the form of tapes. Most preferred arefabrics which are woven from flat tapes, as these have the bestmechanical properties.

[0014] “An accelerated rate of cooling” in this specification meanscooling under conditions such that heat is lost from the monolithicarticle more quickly than if it were cooled from the elevatedtemperature to a predetermined lower temperature below therecrystallisation temperature for the material, under ambientconditions, that is, in still air at ambient temperature, typically 20°C. That is, the monolithic article reaches the lower temperature morequickly than it would under ambient conditions. Cooling is notnecessarily accelerated throughout the temperature range from theelevated temperature to the lower temperature. Suitably, however, anaccelerated rate of cooling may be applied throughout the range from theelevated temperature down to the lower temperature.

[0015] The actual lower temperature is selected depending upon therecrystallisation temperature of the material being treated and shouldbe one sufficiently below the recrystallisation temperature to ensurethe material is prevented from recrystallising once the lowertemperature has been reached. This may be achieved with a lowertemperature as little as 10° C. below the recrystallisation temperaturewhen one employs temperature stabilisation techniques. However it willbe appreciated that a greater temperature differential such as between10° C. and 20° C. or between 10° C. and 40° C. or indeed greater may beemployed to good effect. In the particular example discussed laterherein the lower temperature for polypropylene was selected to be

[0016] An accelerated rate of cooling may in principle be achieved byone or more of the following means:

[0017] contacting the compacted web with a fluid which is below ambienttemperature

[0018] contacting the compacted web with a fluid which is a betterthermal conductor than air at ambient temperature

[0019] providing for relative movement between the compacted web and afluid; most practicably, by impelling the fluid over the compacted web.

[0020] The rate at which the compacted web is cooled is preferably atleast 10° C. per minute, preferably at least 30° C. per minute, stillmore preferably at least 50° C. per minute. Particularly preferred isextremely rapid cooling of at least 100° C. per minute, preferably atleast 200° C. per minute, and in the case of thin sheets in excess of500° C. per minute. These are average values applying to the entirecooling phase, from the elevated temperature to 100° C. Very rapid ratesof cooling may be termed quenching and may, indeed, be achieved by thetraditional quenching method long used in the metallurgical art, ofimmersing the respective article in water.

[0021] The accelerated rate of cooling of the compacted web inaccordance with the present invention only applies down to 100° C.,which is significantly below the recrystallisation temperature.

[0022] It is preferred that the hot compaction process of the inventionuses a compaction pressure not exceeding 10 MPa. It is also preferredthat a single pressure is used throughout the hot compaction process.Most preferred pressures are between 1 and 7 MPa, particularly between 2and 5 MPa. It is preferred that the hot compaction pressure ismaintained during cooling.

[0023] The minimum temperature at which the fibres should be contactedis preferably that at which the leading edge of the endotherm, measuredby Differential Scanning Calorimetry (DSC), of the constrained polymerfibres extrapolated to zero intersects the temperature axis. Preferably,the temperature at which the fibres are compacted is no greater than theconstrained peak temperature of melting at the ambient compactionpressure—i.e. the temperature of which the endotherm reaches it highestpoint. The proportion of the fibres which is melted during the hotcompaction process is generally between 10 and 50 percent by weight.

[0024] Preferably the fibres used in the present invention have a weightaverage molecular weight (M_(w)) in the range 100,000 to 800,000. Incertain embodiments M_(w) is in the range 250,000 to 450,000, forexample 330,000 to 400,000. In certain other embodiments M_(w) is in therange 100,000 to 250,000, for example 150,000 to 220,000; M_(w) beingdetermined by the method described hereinafter. With such materials oflower M_(w) the present invention provides a route to high yield stressand Young's modulus, yet should show a yield point rather than brittlefailure.

[0025] The polymer is preferably a polypropylene homopolymer, but may bea copolymer comprising polypropylene. Generally any copolymer containingpolypropylene such as those disclosed in WO 98/15397 may be used.

[0026] Compaction of the polypropylene may be carried out in anautoclave, or in a belt press or other apparatus in which the assemblyis fed through a compaction zone where it is subjected to the requiredelevated temperature and pressure. Thus, the process may be operated asa continuous or semi-continuous process. Cooling is preferably effectedwhilst the sheet is restrained against dimensional change, for exampleby being held under tension or by being still under a compactionpressure. In the case of a belt press for example, the belt itself maybe cooled (for example using air chilled by ice water) in the regionimmediately after the heating zone. In this way, it is possible toachieve cooling rates of up to 500° C. per minute.

[0027] The monolithic article may be regarded as a polypropylenecomposite made up of a polypropylene matrix phase which was producedduring the process, and a polypropylene fibre phase, a proportion ofwhich may show selective surface melting, arising from the process. Theproperties of both the matrix phase and the fibre phase are ofsignificance in achieving a monolithic article of the requiredproperties, and they may be defined, and studied, separately.

[0028] Preferably the Young's modulus of the matrix phase is at least0.9 GPa, more preferably at least 1.2 GPa, more preferably at least 1.5GPa, and most preferably at least 1.8 GPa.

[0029] Preferably the failure strength of the matrix phase is at least20 MPa, more preferably at least 25 MPa, and most preferably at least 35MPa.

[0030] Preferably the failure strain of the matrix phase is at least 5%.

[0031] Preferably the Young's modulus in the longitudinal direction(which may alternatively be called the draw or axial direction) of thefibre phase is at least 4 GPa, more preferably at least 6 GPa, and mostpreferably at least 8 GPa.

[0032] Preferably the failure strength in the longitudinal direction ofthe fibre phase is at least 250 MPa, more preferably at least 350 MPa,and most preferably at least 420 MPa.

[0033] Preferably the failure strain in the longitudinal direction ofthe fibre phase is at least 5%.

EXAMPLE SET A

[0034] The effect of cooling rate was established by examining thecooling of a completely melted fabric, to simulate the melted matrixphase in a hot compacted sheet. It has been found that the properties ofa hot compacted sheet are a combination of the properties of theoriginal oriented fibres (the reinforcing phase), and the portion of thefibres which are melted (the matrix phase). Therefore by examining theproperties of a melted fabric which has been cooled at different rates,it is possible to simulate the effect of cooling a hot compaction sheetat different rates.

[0035] The fabrics used were made from a number of different melt formedpolypropylene homopolymers detailed in Table 1 below. The reinforcementtype indicates the type of fibre from which the fabric is woven. TABLE 1Polymer No. 1 2 3 4 Reinforcement Multifilament Fibrillated Flat tapeFlat tape type bundles tape Young's modulus 9.5 10.9 6.2 6.8 E(GPa)Failure strength 453 350 370 422 σ_(F) (MPa) Failure strain 12 6 16 16σ_(F)(%) Density 907 912 932 910 ρ (kg/m³) M_(n) 38,500 55,800 56,10078,100 M_(w) 191,000 290,000 325,000 360,000

[0036] M_(w) and M_(n) were measured by Rapra Technology Limited, ofShropshire, UK.

[0037] Details of the testing are as follows: Instrument Waters 150 CVColumns Plgel 2 × mixed bed-B, 30 cn 10 microns Solvent1,2-dichlorobenzene with anti-oxidant Flow-rate 1.0 ml/min (nominal)Temperature 140° C. (nominal) Detector refractive index and differentialpressure GPC system calibrated with polystyrene

[0038] Woven cloths made of each of the above polymers were completelymelted by heating two layers of cloth in a hot press at 200° C. Thepressure applied was 2.8 MPa, although since the samples were completelymelted this was not critical. Cooling was achieved either by removingthe sample and plunging it into water (quenching) or in the hot press bypassing a coolant through the heated platens, after switching off theheating. Depending on the rate of cooling required 100% water, or aircontaining water droplets, was used as the coolant. In this example fastcooling in the press means a cooling rate of 20-30° C./min. The slowestcooling rate, 1-2° C./min, was achieved by just switching off theheating and allowing the assembly to cool naturally in air.

[0039] Samples which were cooled in the press were removed from thepress when the temperature had dropped to 100° C., which is 20° C. belowthe crystallisation temperature measured by DSC. The cooling rate istherefore determined by the time taken to cool from the compactiontemperature down to 100° C.

PHYSICAL PROPERTIES

[0040] Mechanical Properties

[0041] The stress/strain behaviour of the above cooled films wasmeasured using an RDP Howden servo-mechanical tensile testing machine.The tensile tests on the compacted sheets and the melted films werecarried out following ASTM D638 using a dumbbell shaped specimen. Anominal strain rate of 1051 was used for all the tests. The samplestrain during the tests was measured using a Messphysik videoextensometer. Five samples were tested for each material at atemperature of 20+2° C. and a relative humidity of 35±5%.

[0042] Typical stress-strain curves are shown in FIG. 1, for each of thefour polymer tests. The results show that for all four polymers, thequenched samples were ductile and drew in a stable manner with theformation of a stable neck region. Strain for these samples was measuredfrom the crosshead speed, rather than directly on the sample, for if theneck formed outside the measured region, the strain in the measurementregion actually decreased. The traces for the cooled and quenchedsamples have been displaced a small way along the x-axis simply in orderto display each trace clearly.

[0043] For the sample made using fast cooling differences in behaviourwere seen. The lowest molecular weight polymer (Polymer 1, Graph 1)showed an initial linear region, with an increased slope compared to thequenched sample—indicating higher Young's modulus—then a yield point,again higher than the quenched sample, then rupture after 7-8% ofelongation. This form of stress-strain behaviour is often termednecking-rupture. Two intermediate molecular weight samples (Polymers 2and 3, Graphs 2 and 3) showed the formation of a neck but drawing didnot stabilise and rupture occurred at ˜25% (0.25) strain. Only thehighest molecular weight Polymer 4 (Graph 4) showed stable drawingfollowing application of the fast cooling rate.

[0044] All the samples made by slow cooling showed necking-rupture orbrittle behaviour. The failure strains of the original fibres weremostly between 10 and 20% (0.1 and 0.2): therefore if the matrix failsbelow this value then a hot compacted composite would see prematurematrix failure before the reinforcing phase can reach full load carryingcapacity, leading to premature delamination. It is seen that at theslowest cooling rate, none of the polymers reached this desired failurestrain. In particular, the low molecular weight Polymer 1 showed brittlefailure at a low stress.

[0045] It is clear that the cooling rate of the hot compaction processis a key process parameter, because it has a significant effect on themechanical properties of the matrix phase, probably due to changes incrystallinity. The above results show that both quenching and fastcooling enhanced matrix ductility. As explained above, the greater theductility of the matrix phase (and therefore the higher its failurestrain) the better, in many situations, especially those in which theproperties of hot compacted sheet are dominated by the matrix phase.These include interlayer adhesion (or peel strength) which dependsmainly on the properties of the matrix, thermoformability wheresignificant interlayer shear occurs (differential sliding between thelayers), and normal tensile properties.

[0046] Density

[0047] The densities of the original oriented materials and thecompacted sheets were measured using a density column. The column wasmade from a mixture of digycidyl ether and isopropanol to give a densityrange of ˜890 to ˜930 kg/m³. The results are shown in Table 2. TABLE 2Polymer Material and cooling regime Density (kg/m³) 1 Original fibres907 Melted film - quenching 911 Melted film - slow cooling 915 2Original fibres - (cloth D) 912 Melted film - quenching 920 Meltedfilm - slow cooling 924 3 Original tapes (cloth E) 910 Melted film -quenching 920 Melted film - slow cooling 925

[0048] Modulus

[0049] The Young's Modulus was determined in the initial linear regionof the stress strain curve following the guidelines of ASTM D638. Theresults are shown in Table 3 below. TABLE 3 Young's modulus E (GPa) FastSlow Polymer Quenching cooling cooling 1 1.04 ± 0.02 1.85 ± 0.05 2.08 ±0.13 2 1.00 ± 0.03 1.58 ± 0.06 1.71 ± 0.11 3 1.00 ± 0.09 1.24 ± 0.091.33 ± 0.01 4 0.95 ± 0.06 1.22 ± 0.10 1.37 ± 0.08

[0050] Tables 2 and 3 show the density and Young's modulus of thevarious melted films. Both of these properties can be used as a measureof the crystallinity of the films, as one can attribute increases ineither parameter with an increase in crystallinity. As the cooling rateis increased, the density and Young's modulus for each polymer typedecreases. The cause is believed to be a decrease in crystallinity.Another result is an improvement in ductility.

[0051] The present invention is of benefit in situations where improvedductility (or related properties such as peel strength) is required. Itis of particular benefit for polymers of lower M_(w), for example ofM_(w) 250,000 and below, since slow cooling regimes provide very brittleproducts, with such polymers.

EXAMPLE SET B

[0052] Peel Strength

[0053] Sheet samples were made using Polymer 1 cloths, and further sheetsamples were made using Polymer 4 cloths. In each case the sheet sampleswere made of four layers of cloth, in a 0/90/90/0 lay-up (that is, themiddle layers are orthogonal to the outer layers), and conditions werechosen to give a oriented fibre/matrix microstructure. In this way thecentral layers, where the peeling takes place, are in the sameorientation, but overall the sheet samples have balanced properties.

[0054] Appropriate compaction conditions were chosen for the Polymer 1and Polymer 4 samples. For Polymer 1 this was a compaction temperatureof 182° C., a compaction pressure of 2.8 GPa, and a dwell time of 5minutes. For Polymer 4 this was a compaction temperature of 192° C., acompaction pressure of 2.8 GPa, and a compaction dwell time of 5minutes. Slow cooling was achieved by switching off the heating andallowing the heating platens, containing the sheet sample, to cool inair (1-2° C./min). Fast cooling (35° C./min in this example) wasachieved by passing water through the heating platens.

[0055] The sheet samples were then tested for peel strength (interlayeradhesion). The test was the T-Peel test (ASTM D1876). Samples fortesting were 10mm wide and 100 mm long and were tested using a crossheadspeed of 100 mm/min. The testing was carried out parallel to the warpdirection. Three samples were tested for each polymer and the resultswere averaged.

[0056] The results are shown in Table 4 below. TABLE 4 Average peelSample Cooling regime strength (N/10 mm) Polymer 1 Fast cooling 2.75 ±0.66 Slow cooling 1.77 ± 0.32 Polymer 4 Fast cooling 9.1 ± 2.2 Slowcooling 9.5 ± 2.5

[0057] It will be seen that the peel strength values for Polymer 4 are,as expected, higher than the values for Polymer 1. The values forPolymer 4 are essentially the same for the fast and slow cooled samples.In contrast the fast cooled samples of Polymer 1 have a much higher peelstrength value than the slow cooled samples.

1. A process for production of a monolithic article from a web of fibresof oriented polypropylene homopolymer or copolymer, comprising the stepsof subjecting the web to elevated temperature and pressure sufficient tomelt a proportion of the polymer and compact it, and then cooling thecompacted web, wherein an accelerated rate of cooling is employed downto a lower temperature and in which said lower temperature is apredetermined amount below the recrystallisation temperature of thefibres.
 2. A process for production of a monolithic article from a webof fibres of oriented polypropylene homopolymer or copolymer, comprisingthe steps of subjecting the web to elevated temperature and pressuresufficient to melt a proportion of the polymer and compact it, and thencooling the compacted web, wherein an accelerated rate of cooling isemployed down to 100° C.
 3. A process as claimed in claim 1 wherein thecompacted web is cooled at a mean rate of at least 10° C. per minute,down to 100° C.
 4. A process as claimed in claim 2 wherein the compactedweb is cooled at a mean rate of at least 30° C. per minute, down to 100°C.
 5. A process as claimed in claim 1 wherein the compaction pressuredoes not exceed 10 MPa.
 6. A process as claimed in claim 1, wherein thefibres are melt formed fibres.
 7. A process as claimed in claim 1wherein the weight average molecular weight (Me) of the fibres is in therange 100,000 to 800,000.
 8. A proccss as claimed in claim 7 wherein theweight average molecular weight (M_(w)) of the fibres is in the range100,000 to 250,000.
 9. A process as claimed in claim 8 wherein theweight average molecular weight (M_(w)) of the fibres is in the range150,000 to 220,000.
 10. A monolithic article manufactured by a processas claimed in claim 1, having a matrix phase of polymer which wasproduced by selective melting of the oriented phase during the processand an oriented fibre phase a fraction of which was melted during theprocess.
 11. A monolithic article as claimed in claim 10 wherein theYoung's modulus of the matrix phase is at least 0.9 GPa.
 12. Amonolithic article as claimed in claim 10 wherein the failure strengthof the matrix phase is at least 20 MPa.
 13. A monolithic article asclaimed in claim 10, wherein the failure strain of the matrix phase isat least 5%.
 14. A monolithic article as claimed in claim 10 wherein theYoung's modulus in the longitudinal direction of the oriented fibrephase is at least 4 GPa.
 15. A monolithic article as claimed in claim 10wherein the failure strength in the longitudinal direction of theoriented fibre phase is at least 250 MPa.
 16. A monolithic article asclaimed in claim 10 wherein the failure strain in the longitudinaldirection of the oriented fibre phase is at least 5%.
 17. A process offabricating a monolithic article comprising polypropylene polymer orcopolymer, or a monolithic article thus formed, substantially ashereinbefore described with particular reference to the accompanyingexamples.